The invention relates to cermet powder for thermal spray coating, which is applied to an inside-furnace roll for conveying a heat-treated material in a heat treating furnace such as a continuous annealing furnace for steel strips and has excellent build-up resistance property and excellent oxidization resistance property. The invention also relates to a spray coated roll on whose surface the cermet powder for thermal spray coating has been sprayed and which is provided inside a furnace (which roll will be referred to as an xe2x80x9cinside-furnace rollxe2x80x9d or a xe2x80x9cspray coated rollxe2x80x9d hereinafter).
When a steel strip is continuously annealed, the steel strip is conveyed in an oxidizing or reducing atmosphere at the temperature of 600 to 1300xc2x0 C. A number of heat resistant rolls is provided in the face such that the rolls are used as inside-furnace rolls and the steel strip is supported by these rolls. However, after being continuously used for a long time, oxides of Mn, Si, Al etc. and the like which are oxides attached on the steel strip (or scale) etc. are deposited and accumulated on the surface of the inside-furnace rolls, whereby what is called xe2x80x9cbuild-upxe2x80x9d is formed. When such build-up is formed, the build-up may generate scars on the surface of the steel strip and thus cause deterioration of quality thereof. Accordingly, the operation must be stopped immediately and the surfaces of the inside-furnace rolls have to be cleaned in a dummy material or the like, or in a worse case, the furnace must be opened, so that grinding or the like of the surfaces of the inside-furnace rolls or exchange of the rolls can be carried out.
Due to this, in order to prevent the accumulation of build-up on the surface of inside-furnace rolls, an invention in which a thermal sprayed coating is formed on the roll surface has been proposed. Several types of such rolls have already been used in practice. However, none of them can completely prevent the build-up yet.
As shown in FIG. 1, rows of build-up 3 are formed in the circumferential direction of the roll, in parallel with each other, along the portion of the roll on which a metal strip 1 is conveyed.
The build-up 3 has a sectional configuration as shown in FIG. 2. FIG. 2 shows a state in which the build-up 3 has been formed on a roll surface of an inside-furnace roll 2, i.e., on a thermal sprayed coating 2a formed on a roll base material 2b. 
The inventions relating to a thermal sprayed coating film which have been disclosed will be described hereinafter.
(1) Unexamined Publication No. JP-2-270955 A
A hearth roll for a high temperature heat treating furnace having a cermet thermal spray coating film formed thereon, the cermet thermal spray coating film being made of an alloy containing 5 to 20 wt. % of Cr2O3xe2x80x94Al2O3 and 95-80 wt. % of Coxe2x80x94Nixe2x80x94Crxe2x80x94Alxe2x80x94Y
(2) Unexamined Publication No. JP-63-199857 A
A high-temperature-resistant spray coated member having a cermet thermal spray coating provided thereon, the cermet thermal spray coating being composed of 51 to 95 vol. % of Al2O3 and MCrAlY (M is an alloy made from the compounds selected from the group consisting of Fe, Ni, Co and Si)
(3) Unexamined Publication No. JP-63-47379 A
An inside-furnace roll for a heat processing furnace, in which the surface layer of a cermet thermal spray coating layer composed of 30 to 80 wt. % of ZrSiO4 and MCrAlY (M is an alloy made from the compounds selected from the group consisting of Fe, Ni, Co and Ta) is coated with chromium oxide
(4) Unexamined Publication No. JP-60-56058 A
A hearth roll having a spray coated layer of Al2O3xe2x80x94MgO as the roll uppermost layer, in addition to at least two intermediate layers provided between the uppermost layer and the roll parent material and formed by spraying of a mixture of Al2O3xe2x80x94MgO and a bonding metal
(5) Unexamined Publication No. JP-3-226552 A
A thermal spray coating material composed of 5 to 50 vol. % of boride and MCrAlY (M is Fe or Ni or Co) and a coated article having a thermal spray coating film made of the thermal spray material
(6) Unexamined Publication No. JP-8-67960 A
A cermet thermal spray material produced by mixing at least one type of powdery, refractories, which exhibits low reactivity against manganese oxide, is selected from the group consisting of MgAl2O4, Y2O3 and MgO, and of which compound content is in the range of 5 to 90 wt. %, with MCrAlY (M is Fe or Ni or Co), and a hearth roll using the same
(7) Unexamined Publication No. JP-7-11420 A
A cermet film and a roll for a heat treating furnace which contain: 1 to 60vol. % of at least one type boride selected from the group consisting of CrB2, ZrB2, WB, TiB2 and the like; 5 to 50 vol. % of at least one type of carbide selected from the group consisting of Cr3C2, TaC, WC, ZrC, TiC, NbC and the like; and metal (for example, MCrAlY) which substantially constitutes the residual.
Here; MCrAlY generally represents a heat resisting alloy in which Cr, Al and Y are added, each by an appropriate amount, to a base material which is at least one type of compound selected from the group consisting of F, Ni and Co.
The inventions of the aforementioned (1)-(7) exhibit not a little build-up reducing effect, in the heat treating of an ordinary, common steel strip. If the heat treating includes treating of high tensile strength steel material (the steel material which normally exhibits tensile strength of no lower than 340 MPa in the state of a cold rolled steel plate and tensile strength of no lower than 440 MPa in the state of a hot rolled steel plate) to some extent, such treating of high tensile strength steel material generally does not cause a problem, as long as the amount of the high tensile strength steel material to be processed is small.
However, in recent years, as the use of high tensile strength steel material increases, the measurement as described above can no longer be so effective for prevention of build-up.
Specifically, the high tensile strength steel material contains a larger amount of Mn (0.6 to 3.5 mass %), Si (no more than 2 mass %) and the like than ordinary steel materials do and these elements tend to appear, in a condensed state, on the surface of the steel material during the heat treating process, whereby a relatively large amount of Mn oxides are formed on the surface of the steel strip. Due to this, when a relatively large amount of high tensile strength steel material is heat treated, the build-up resistance property which is more excellent than the conventional level is required of the inside-furnace roll.
The present invention has an object to provide long-durability cermet powder for thermal spray coating, which has excellent build-up resistance property, has excellent oxidization resistance property required of an inside-furnace roll and thus solves the aforementioned problems, and to provide a thermal spray coated roll inside-furnace roll in which the aforementioned cermet powder is applied.
In short, the present invention has solved the aforementioned problem by the cermet powder for thermal spray coating or the thermal spray coated roll as described below.
(1) A cermet powder for thermal spray coating, which is used for a conveyer roll inside a heat treating furnace for a steel strip, comprising:
an alloy powder containing 3 to 8 mass % of Al, 16 to 25 mass % of Cr, 0.1 to 1 mass % of Y, and at least one of Co and Ni as the residual, with respect to the whole amount of the cermet powder; and
a ceramic powder containing at least one of 1 to 5 mass % of a boride and 5 to 10 mass % of a carbide, with respect to the whole amount of the cermet powder.
(2) A cermet, powder for thermal spray coating described in the aforementioned (1), wherein the cermet powder contains ceramic powder of at least one type of rare earth oxide selected from the group consisting of Y2O3, La2O3 and CeO2, by the total amounts of the compounds of no less than 10 mass % with respect to the whole amount of the cermet powder.
(3) A cermet powder for thermal spray coating described in the aforementioned (1) or (2), wherein the cermet powder contains 1 to 25 mass % of the ceramic powder, with respect to the whole amount of the cermet powder.
(4) A cermet powder for thermal spray coating described in the aforementioned (1), wherein the boride contains at least one compound selected from the group consisting of ZrB2, CrB, TiB, MoB, by the total content of the boride of 1 to 5 mass %.
(5) A cermet powder for thermal spray coating described in the aforementioned (1), wherein the carbide contains at least one compound selected from the group consisting of Cr3C2, TiC, NbC, TaC, by the total content of the carbide of 5 to 10 mass %.
(6) A thermal spray coated roll, characterized in that it has a thermal sprayed coating formed on a roll surface thereof, the coating being formed by thermal spraying the cermet powder for thermal spray coating of any of the aforementioned (1) to (3) on the roll surface.
(7) A thermal spray coated roll of the aforementioned (6), wherein the thermal spray coated roll is a conveyer roll inside a heat treating furnace in which furnace a high tensile strength steel plate is conveyed.